Optical projection displays often use polarizing elements to change the polarization of the light that they receive. One type of polarizing element is an active polarizing switch, which is also called a switchable polarizer. A switchable polarizer receives electrical signals that control how the polarizer changes the polarization of the light.
FIG. 1 presents one prior art switchable polarizer. This polarizer 100 includes two layers of glass 105 and 110, two transparent indium-tin-oxide (“ITO”) electrodes 115 and 120, a layer of liquid crystal material 125, and two contacts 130 and 135. The liquid crystal material 125 changes the polarization of the incident light based on a variety of criteria. These criteria include the structure and orientation of the liquid crystal material 125 as well as the potential difference across this material.
The potential difference across this material 125 is a primary influence on its polarization. This is because the molecular orientation of the liquid crystal material 125 rotates when an electric field is applied across it. This rotation, in turn, modifies how this material changes the polarization of the light passing through it. A potential difference is applied across the liquid crystal material by placing the first electrode 115 (via contact 130) at a first potential (V1) and placing the second electrode 120 (via contact 135) at a second potential (V2) that is different from the first potential. Hence, applying two different voltages to the electrodes 115 and 120 modifies how the liquid crystal material 125 changes the polarization of the light.
The operation of switchable polarizer 100 is also partially dependent on the temperature. Specifically, the angle that this switch rotates the polarization of the incident light varies with the temperature. Consequently, the temperature of the switchable polarizer needs to be controlled to ensure that it rotates the polarization of the incident light by a precise amount. Otherwise, imprecise polarization by the polarizer 100 will degrade the brightness and contrast of the displayed image.
One prior art solution for controlling the temperature of the switchable polarizer 100 is to glue a heater to the polarizer. This heater controls the temperature of the polarizer. This solution, however, makes the structure of the switchable polarizer somewhat bulky and adds to the expense of this device. This solution also makes it harder to manufacture this device reliably. In addition, this prior art solution results in uneven temperature control since the heater cannot be placed in the optical path.
FIG. 2 presents another prior art solution for controlling the temperature of switchable polarizers. Like the switchable polarizer 100 of FIG. 1, the switchable polarizer 200 of FIG. 2 includes (1) a layer of liquid crystal material 125, (2) two ITO electrodes 115 and 120 that surround the liquid crystal material and establish the electric field across this material, (3) two glass layers 105 and 110, and (4) two contacts 130 and 135.
Polarizer 200, however, also includes (1) a third ITO electrode 205 for heating the polarizer, and (2) a third glass layer 210 for protecting the third ITO layer 205. To generate heat, one end of the third electrode is placed at a third potential (V3) while the other end of this electrode is placed at a fourth potential (V4) different from the third potential. The potential difference across the third electrode 205 (i.e., the ΔV, which equals V3-V4) causes a current (I) to flow through this electrode. The current flow will cause power to dissipate at a rate of I2RL, where R is the resistance of the third ITO electrode per unit length and L is its length. This power is dissipated in form of heat.
This prior art solution has several disadvantages. One disadvantage is that the potential difference across the third ITO layer results in a potential gradient across the switch 200. This potential gradient causes polarization of the light to be non-uniform across the switch.
Another disadvantage of the switch 200 is that the third electrode 205 attenuates the intensity of the light. Each layer of ITO attenuates four to six percent of the light depending on the wavelength of the light. The light is attenuated the most in the blue range, which often is one of the more important color components. The attenuation of the light, in turn, degrades the brightness and contrast of the display system.
Therefore, there is a need in the art for high performance switchable polarizers for optical projection displays. There is also a need for switchable polarizers that have thermal control structures which do not introduce voltage gradients across the polarizers. There is also a need for switchable polarizers with thermal control structures that do not attenuate the intensity of the incident light.